Many communications systems, especially those intended for use in mobile platforms, must be environmentally robust both in terms of their hardware and signaling format. Airborne communication systems used with a plurality of UHF line-of-sight and satellite links, for example, may incorporate a transceiver mounting chassis or enclosure containing diverse communication equipment such as RF transmitter modules, RF receiver modules and various digital signal processing modules which control operation of the RF components and interface digital communications signals with attendant encryption and decryption circuits. Considering that each communication link has its own dedicated signaling scheme, suppliers of this equipment typically provide each system as an integrated unit.
One of the standard architectures employed by suppliers of such systems is the Versa Module Europa or VME bus. RF signaling circuits and digital signaling modules plug into discrete connector slots on the VME bus to avoid cross-talk and provide isolation between such components, and, to conform with the relatively tight dimensional spacing between the connector slots on the VME bus. Whether the communication system is intended for use in a mobile platform as noted above, or other applications, the enclosure or chassis which houses the VME bus and communication equipment components must be designed to withstand harsh environmental conditions including vibration, temperature variations and exposure to foreign matter. Consequently, VME bus specifications mandate ruggedized housing architectures which have the ability to cool circuit components and protect them from exposure to excess vibration and foreign material.
Initial efforts to meet VME bus specifications included chassis designs incorporating expensive and complex heat transfer elements. Alternatively, or in addition to these measures, the circuit card assemblies were provided with special, thermally robust circuit elements which added cost and unwanted bulk to the design.
These deficiencies were addressed to some extent in the system disclosed in U.S. Pat. No. 5,835,349 to Giannatto et al. This patent discloses a housing and cooling assembly which reduces the cost and overall size of the unit, while providing effective cooling of circuit components on the circuit card assemblies. A “U-pass” heat exchanger is mounted directly to each individual circuit card assembly which imparts structural rigidity to the cards, and isolates the circuit cards from the flow of cooling fluid, e.g. air, passing through the heat exchanger in a U-shaped flow path to and from an inlet/exhaust plenum. The circuit card assembly of each circuit card/heat exchanger combination or module is plugged into the VME bus, while the inlet and outlet of the heat exchanger is sealed with a gasket to elements of the chassis.
While the system of the U.S. Pat. No. 5,835,349 patent provides a number of advantages over prior approaches, it nevertheless has some limitations. Cooling air from outside of the heat exchanger circulates from the inlet of the plenum to the opposite end of the heat exchanger, and then reverses direction in order to flow to the exhaust portion of the same plenum. This U-shaped flow path creates a relatively large pressure drop that reduces the heat transfer performance of the heat exchanger. Additionally, the use of a gasket to seal the inlet and outlet of the heat exchanger reduces the reliability of the system, and creates a maintenance issue since the gaskets can be easily damaged and may require periodic replacement. Further, the circuit card/heat exchanger modules are provided with rails at each end which engage opposed slots formed in the end walls of the chassis to mount them in place. In order to readily permit installation of the modules in the chassis, the mating rails and slots cannot be constructed with tolerances which are too tight, and therefore the overall rigidity of the assembly is sacrificed to some extent and tolerance to vibration is reduced.